An outboard motor may sometimes be operated in muddy or agitated waters in which fine sand particles or the like are dispersed or mixed. Consequently, the water pump for cooling the engine with such water must be capable of withstanding the adverse effects of these particles. For this purpose, the impeller is frequently made of an elastic material such as rubber. This tends to resist wear that would be caused by interaction with the foreign substances, to prevent the wall of the pump chamber from cracking due to the pressure of the foreign substances. In order to fix the impeller of such elastic material to a drive shaft, a bush having a greater rigidity is customarily buried in the boss of the impeller, and the drive shaft is inserted into the bush, and the bush and the drive shaft are keyed to each other. This jointing construction creates a clearance between the bush and the drive shaft. As a result, there has been the disadvantage that muddy water steals into the clearance so that its foreign substances wear the bush, the drive shaft, the key and the key way. It is an object of this invention to minimize or prevent this disadvantage.